Method for vacuum pressure forming reinforced plastic articles

ABSTRACT

A hollow, reinforced, molded automotive hard top is made using a sheet of thermoplastic material with nanoparticles dispersed therein. The particles comprise less than 15% of total volume of the plastic sheet, at least 50% of the particles have a thickness less than about 20 layers, and at least 99% of the particles have a thickness of less than about 30 layers. The sheet is preheated and molded in an assembly having mold surfaces corresponding to the hard top configuration. Vacuum is applied to one side of the assembly and pressurized gas is applied to the opposite side of the assembly to force the sheet into conformity with the mold surfaces. After cooling, the conformed sheet is transferred to another mold assembly, where a reinforced plastic melt having a blowing agent is applied. Together, the solidified melt and the conformed plastic sheet form the hard top.

[0001] This patent application is a division of application Ser. No.09/857,768, filed Jun. 11, 2002, which is the National Phase ofInternational Application PCT/US99/29987 filed Dec. 17, 1999, whichclaims priority from U.S. Provisional Application No. 60/113,064, filedDec. 21, 1998. The contents of these applications are herebyincorporated into the present application by reference in full.

FIELD OF THE INVENTION

[0002] The present invention relates to vacuum forming or pressureforming articles and apparatuses, and, more particularly, a moldingmethod combining vacuum and pressure for producing reinforcedthermoplastic articles. The invention also relates to molded articleshaving reinforced foam fillers.

BACKGROUND OF THE INVENTION

[0003] Traditional blow molding is limited as to the wall thickness ofthe article to be formed, as well as the complexity of article shape. Toovercome this, thermoforming, a modification of blow molding, cansuffice for manufacturing articles having relatively thick walls and/orcomplex shapes. Thermoforming processes such as plug assisted vacuumforming or pressure forming permit the production of items having a wallthickness of up to about ⅜ inch (95.25 mm). Articles formed byconventional blow molding, by contrast, are usually limited to wallthicknesses of less than about ⅛ inch (31.75 mm). This is due, in part,to the negative effects exerted on the blowing process by the greatervolumes of polymer resin required to achieve thicker walls. For example,increasing amounts of viscous molten polymer will limit the size, wallthickness and complexity of an article to be formed, as blown airbecomes progressively ineffective at expanding molten polymer as thevolume of polymer material increases.

[0004] In basic vacuum forming, a carrier frame delivers a heatedplastic sheet to a mold assembly, after which the sheet is clamped andsealed against the mold edge surfaces. Application of a vacuum causesatmospheric pressure to force the sheet against the mold cavity toassume the cavity shape. Mold cooling promotes the formation of a thinsheet having the dimensions defined by the mold.

[0005] As a variation of blow molding, the above-mentioned processfurther includes the step of blowing air of controlled pressure to forcethe heated sheet away from the cavity into a bubble. A shaped plug isthen inserted into the bubble, pressing the bubble back into the moldcavity after the sheet has been sealed across the mold cavity. Uponreaching the bottom of the mold cavity, compressed air and/or a vacuumis applied to force the sheet against the mold. After forcing the sheetinto the cavity, a full vacuum is applied from the cavity side andpositive pressure is applied from the plug side of the apparatus tocomplete the formation of a molded article. After it has solidified, themold assembly is opened, and the article is removed.

[0006] In a similar fashion, drape forming entails either draping aplastic sheet over or moving a male mold into a plastic sheet, andthereafter clamping, heating, and sealing the sheet over the male mold.Numerous vent holes in the mold apparatus permit a vacuum to be drawn,allowing atmospheric pressure to force the draped sheet into thecontours of the mold cavity. Upon cooling, the sheet shrinks onto themold.

[0007] Typical vacuum-formed or pressure-formed products include blisterand skin packaging, food and drink containers, toys, luggage, and autoand appliance parts. Polystyrene, polypropylene, HDPE, thermoplasticpolyester, ABS and vinyls are often used to manufacture these articles.Films and sheets formed in this fashion are often laminated by melt oradhesive processes to enhance their functional performance.

[0008] A need has arisen for reinforced blow molded articles having goodthermoinsulating and sound barrier properties. In particular, theresurgence in popularity of removable hard tops and T-tops forautomobiles has prompted engineers to seek better insulatingcharacteristics of blow molded articles. For example, lightweight,suitably thermoinsulated removable hard tops for sport utility vehicles(SUVs) are in high demand by consumers. While blow molding provides forsufficiently lightweight automobile parts, combining the suitable weightproperties with good impact resistance and thermoinsulating propertieshas heretofore been difficult.

[0009] The usefulness of blow molding techniques for forming such impactresistant, thermoinsulated articles has not been practical due to thestructural characteristics of the plastic material conventionally usedin blow molding. That is, the ability to blow mold light weight,thermoinsulated parts is limited by the fact that the parts produced canbe only so large or so thin before the parts lose their structuralintegrity and impact resistance.

[0010] Further, most insulating materials must be laminated to the partafter blow molding into the desired shaped. For example, urethane foammay be introduced to a blow molded part to improve insulatingcapabilities, as well as dimensional stability. However, this process isplagued by incompatibility between the skin component of the molded partand the insulating foam filler. Expensive thermoplastic skins are oftenchemically incompatible with traditional foam insulating materials,preventing strong bond formation within laminated structures. Thus, blowmolded articles having skin and foam fillers of different materials areprone to delamination. A solution to the delamination problem is to fillthe article with a foamed resin identical to the resin used to form theexterior skin of the article. Although this expensive concept isacceptable for many blown articles, it is insufficient for producing acost effective automobile part having good impact resistance.

[0011] Blow molded articles such as sport utility vehicle (SUV) hardtops require good thermoinsulation while exhibiting strong impactresistance. By nature, structural foams lack good impact resistance dueto their open cellular conformation. Thus, blow molded automobile partshaving structural foam insulating materials compatible with an exteriorresin skin require reinforcement.

[0012] Heretofore, in order to reinforce various plastic parts, suchparts would conventionally comprise resins fortified by mineral fillersor glass fibers. However, such reinforcement cannot be used effectivelyin blow molding operations, because the glass fibers limit parisonexpansion characteristics and also have a deleterious effect on the blowmolding assembly itself. Furthermore, such reinforcement has adeteriorating effect on the foaming capabilities of resins. Thus, blowmolded articles having a structural foam component subjected toconventional reinforcement often lack uniform strength and impactresistance.

[0013] Similarly, thermoformed articles having foam backing typicallylack satisfactory levels of impact resistance due to both the need foran aesthetically pleasing skin and the open cellular nature ofreticulated foam. Exterior skin appearance deteriorates with increasingamounts of conventional reinforcing materials. Typical reinforcingmaterials tend to impair the formation of reticulated cells duringblowing of foam resins. Because structural foams are not adequatelyreinforced by conventional means, thermoformed articles comprising goodquality skins laminated to foam backing have inadequate strength andimpact resistance.

SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to overcome the problemsnoted hereinabove. In achieving this object, the present inventionprovides a method for thermoforming reinforced, insulated thermoplasticparts. Accordingly, the present invention provides a method for moldingarticles, comprising the steps of providing a first reinforced plasticsheet comprising at least one thermoplastic material and reinforcementnanoparticles dispersed within the at least one thermoplastic material.The reinforcement particles comprising less than 15% of a total volumeof the plastic sheet, and at least 50% of the reinforcement particleshaving a thickness of less than about 20 layers, and at least 99% of thereinforcement particles having a thickness of less than about 30 layers.The heated plastic sheet is communicated to a first mold assembly havinga first mold cavity defined by mold surfaces. The mold surfacescorrespond to a configuration of the article to be molded. An amount ofthe plastic sheet is communicated to the first mold assembly beingsufficient to form a skin of the article. A vacuum is applied to oneside of the first mold assembly while concurrently applying pressurizedgas to an opposing side of the first mold assembly so as to force theheated plastic sheet into conformity with the mold surfaces. Theconformed plastic sheet is then cooled. The conformed plastic sheet isthen transferred to a second mold assembly. A reinforced plastic meltmade from material identical or different from that of the plastic sheetis introduced to the conformed plastic sheet. The plastic melt has ablowing agent to achieve volume expansion and the production of acellular reticulate structure. The plastic melt is then cooled to form asolidified plastic member adhered to the conformed plastic sheet. Theconformed plastic sheet and the adhered solidified plastic membertogether comprise the article. The article is removed from the secondmold assembly.

[0015] It is also an object of the invention to produce reinforced partsfor automotive applications via plug assisted thermoforming, which hasheretofore been impractical.

[0016] An embodiment of the invention is a child safety seat having areinforced outer skin member and a reinforced foamed structural member.The seat members are formed from at least one thermoplastic material andreinforcement nanoparticles dispersed within the at least onethermoplastic material. The reinforcement particles comprise about 2% toabout 15% of a total volume of the molded hard top, at least 50% of thereinforcement particles have a thickness of less than about 20 layers,and at least 99% of the reinforcement particles have a thickness of lessthan about 30 layers.

[0017] In another embodiment, a substantially hollow molded hard top foran automobile which is filled with foamed insulating material is formedfrom at least one thermoplastic material and reinforcement nanoparticlesdispersed within the at least one thermoplastic material. Thereinforcement particles comprise about 2% to about 15% of a total volumeof the molded hard top, at least 50% of the reinforcement particles havea thickness of less than about 20 layers, and at least 99% of thereinforcement particles have a thickness of less than about 30 layers.

[0018] Other objects and advantages of the present invention will becomeapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] A preferred embodiment of the present invention is describedherein with reference to the drawing wherein:

[0020]FIG. 1 shows a perspective view of a sport utility vehicle hardtopcontemplated by the invention, and

[0021]FIG. 2 shows a sectional view of the top depicted in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] It is contemplated that reinforced skins according to theinvention may be prepared using any conventional pressure formingmethod. Preferably, the mold assembly is provided with appropriate watercooling lines and a temperature control unit in conventional fashion forregulating the temperature of the mold assembly. The molds may assume acomplex or detailed shape, providing for reinforced complex shapeshaving a reinforced foam core produced according to the invention.

[0023] In accordance with the present invention, the plastic melt (andthus the resultant part) comprises at least one thermoplastic materialand reinforcement particles dispersed within the at least onethermoplastic material. The reinforcement particles about 2% to about15% of a total volume of the plastic melt, at least 50% of thereinforcement particles have a thickness of less than about 20 layers,and at least 99% of the reinforcement particles have a thickness of lessthan about 30 layers. The reinforcement filler particles, also referredto as “nanoparticles” due to the magnitude of their dimensions, eachcomprise one or more generally flat platelets. Each platelet has athickness of between 0.7-1.2 nanometers. Generally, the average plateletthickness is approximately 1 nanometer thick. The aspect ratio for eachparticle, which is the largest dimension divided by the thickness, isabout 50 to about 300.

[0024] The platelet particles or nanoparticles are derivable from largerlayered mineral particles. Any layered mineral capable of beingintercalated may be employed in the present invention. Layered silicateminerals are preferred. The layered silicate minerals that may beemployed include natural and artificial minerals. Non-limiting examplesof more preferred minerals include montmorillonite, vermiculite,hectorite, saponite, hydrotalcites, kanemite, sodium octosilicate,magadiite, and kenyaite. Mixed Mg and Al hydroxides may also be used.Among the most preferred minerals is montmorillonite.

[0025] To exfoliate the larger mineral particles into their constituentlayers, different methods may be employed. For example, swellablelayered minerals, such as montmorillonite and saponite are known tointercalate water to expand the inter layer distance of the layeredmineral, thereby facilitating exfoliation and dispersion of the layersuniformly in water. Dispersion of layers in water is aided by mixingwith high shear. The mineral particles may also be exfoliated by ashearing process in which the mineral particles are impregnated withwater, then frozen, and then dried. The freeze dried particles are thenmixed into molten polymeric material and subjected to a high sheermixing operation so as to peel individual platelets from multi-plateletparticles and thereby reduce the particle sizes to the desired range.

[0026] The extruded plastic sheet utilized in accordance with thepresent invention is prepared by combining the platelet mineral with thedesired polymer in the desired ratios. The components can be blended bygeneral techniques known to those skilled in the art. For example, thecomponents can be blended and then melted in mixers or extruders.Preferably, the plastic melt is first manufactured into pellet form. Thepellets are then plasticized in the extruder to form a plastic melt,which exits the extruder in sheet form.

[0027] Additional specific preferred methods, for the purposes of thepresent invention, for forming a polymer composite having dispersedtherein exfoliated layered particles are disclosed in U.S. Pat. Nos.5,717,000, 5,747,560, 5,698,624, and WO 93/11190, each of which ishereby incorporated by reference. For additional background, thefollowing are also incorporated by reference: U.S. Pat. Nos. 4,739,007and 5,652,284.

[0028] Preferably, the thermoplastic used for the purposes of thepresent invention is a polyolefin or a blend of polyolefins. Thepreferred polyolefin is at least one member selected from the groupconsisting of polypropylene, ethylene-propylene copolymers,thermoplastic olefins (TPOs), and thermoplastic polyolefin elastomers(TPEs).

[0029] The exfoliation of layered mineral particles into constituentlayers need not be complete in order to achieve the objects of thepresent invention. The present invention contemplates that at least 50%of the particles should be less than about 20 nanometers in thicknessand, thus, at least 50% of the particles should be less than about 20layers thick. In addition, at least 99% of the reinforcement particlesshould have a thickness of less than about 30 nanometers, which is about30 layers stacked in the thickness direction. With this extent ofexfoliation, with a loading of less than 15% by volume, the benefits ofthe nanoparticles begin to accrue with meaningful effect for many largethin part applications. For example, such loading of nanoparticles willprovide a desired increase in the modulus of elasticity by about 50-70%over conventional fillers. Preferably, about 2% to about 15%, even morepreferably about 2% to about 8% loading in used to achieve desirablereinforcement.

[0030] More preferably, at least 50% of the particles should have athickness of less than 10 nanometers. At this level, an additionalincrease of about 50-70% in the modulus of elasticity is achieved incomparison with the 50% of particles being less than 20 layers thick asdiscussed above. This provides a level of reinforcement and impactresistance that would be highly suitable for most motor vehicle partapplications, such as reinforced insulated hard tops.

[0031] Preferably, at least 70% of the particles should have a thicknessof less than 5 layers, which would achieve an additional 50-70% increasein the modulus of elasticity in comparison with the 50% of less than 10layer thickness exfoliation discussed above. This provides idealreinforcement and impact resistance for large thin parts that mustwithstand substantial impact. It is always preferable for at least 99%of the particles to have a thickness of less than about 30 layers, asparticles greater than this size act as stress concentrators.

[0032] It is most preferable to have as many particles as possible to beas small as possible, ideally including only a single platelet.

[0033] As noted above, the preferred aspect ratio (which is the largestdimension divided by the thickness) for each particle is about 50 toabout 300. At least 80% of the particles should be within this range. Iftoo many particles have an aspect ratio above 300, the material becomestoo viscous for forming parts in an effective and efficient manner. Iftoo many particles have an aspect ratio of smaller than 50, the particlereinforcements will not provide the desired reinforcementcharacteristics. More preferably, the aspect ratio for each particle isbetween 100-200. Most preferably, at least 90% of the particles have anaspect ratio within the 100-200 range.

[0034] Generally, in accordance with the present invention, the plasticmelt and hence the parts to be manufactured should contain less than 15%by volume of the reinforcement particles of the type contemplatedherein. The balance of the part is to comprise an appropriatethermoplastic material and suitable additives. If greater than 15% byvolume of reinforcement filler is used, the viscosity of the compositionbecomes too high and thus difficult to mold.

[0035] By utilizing plastic melt with the loading of nanoparticlesdiscussed above (e.g., less than 15% of a total volume of the plasticmelt), higher modulus of elasticity of conventional large plastic partscan be achieved, and thus be manufactured with a reduced wall thicknesswhile maintaining the same required impact resistance. For example, themodulus of the material used to form an article may be increased tobetween about 200,000 to about 500,000 PSI (1378-34.46 MPa).

[0036] In accordance with the present invention, addition of theexfoliated platelet material as set forth above permits the modulus ofvacuum formed articles to be increased without significantly losingimpact resistance. Because the modulus is increased, large parts, suchas removable automobile hard tops, can be made thinner than what wasotherwise possible. Such parts may also be insulated by reinforced foam,thereby adding sound proofing and thermal insulation to thinner hardtops without jeopardizing impact resistance. More specifically, hardtops for automobiles must have sufficient impact resistance or toughnessto withstand various standard automotive impact tests, particularly rollover tests.

[0037] For example, an automotive hard top must withstand a typicalimpact test wherein the hard top will not crack or permanently deformupon impact. In a conventional IZOD impact test, it is desirable for thepart to withstand at least 10-ft pounds/inch (535 J/m) at roomtemperature and at least 5-ft pounds/inch (263 J/m) at −30° C. In orderto withstand cracking at such force levels, the modulus of aconventional automotive material is typically between about 70,000 toabout 150,000 pounds per square inch (PSI) (482-1034 MPa). In accordancewith the present invention, the hard top modulus can be increased by afactor of 2 to 3 times, without significantly effecting the impactresistance.

[0038] In addition to the above mentioned benefits, use of thenanoparticle reinforced plastic melt enables the coefficient of linearthermal expansion to be reduced to less than 40×10⁻⁶ inches of expansionper inch of material per degree Fahrenheit (IN/IN)/° F., or 72×10⁻⁶mM/mm/°C., which is less than 60% of what was previously achievable forthermoplastic motor vehicle parts that meet the required impact tests.

[0039] As a further benefit, the surface toughness of the hard top canbe improved. The improved surface toughness provided by thenanoparticles greatly reduces handling damage and part scrap. This is asignificant benefit to a part which by design is repeatedly removed froman automobile and must endure unexpected scraping, dropping andnon-collision impact.

[0040] In addition, it is possible to more than double the modulus ofpolymers without significantly reducing toughness. Thus, it is possibleto produce articles like hard tops using 20-35% thinner wall sectionsthat will have comparable performance. The use of nanoparticles canprovide the mechanical, thermal, and dimensional property enhancements,which are typically., obtained by adding 20-50% by weight of glassfibers or mineral fillers or combinations thereof to polymers. However,only a few percent of nanoparticles are required to obtain theseproperty enhancements.

[0041] As a result of the fact that such low levels of nanoparticles arerequired to obtain the requisite mechanical properties, many of thetypical negative effects of the high loadings of conventionalreinforcements and fillers are avoided or significantly reduced. Theseadvantages include: lower specific gravity for a given level ofperformance, better surface appearance, toughness close to that of theunreinforced base polymer, and reduced anisotropy in the molded parts.

[0042] It is preferable for these articles to have reinforcementparticles of the type described herein comprising about 2% to about 8%of the total volume of the article, with the balance comprising thethermoplastic substrate. It is even more preferable for removable hardtops to have reinforcement particles of the type contemplated hereincomprising about 3%-5% of the total volume of the part.

[0043] In accordance with another specific embodiment of the presentinvention, it is contemplated that the blow molding apparatus can beused to make relatively large, highly reinforced parts having a modulusof elasticity of 1,000,000 (6892 MPa) or greater. Conventionally, theseparts typically require loadings of 25-60% by volume of glass fiberreinforcement. This amount of glass fiber loading would result in a highviscosity of any melt pool that could be used in the blow moldingapparatus of the present invention and would thus render the blowmolding apparatus largely impractical for such application.

[0044] Sheets of the plastic melt described above enable the plugassisted thermoforming of large parts having impact resistancecharacteristics previously unattainable. For example, the thermoformingsystem of the present invention is able to manufacture relatively largearticles having a modulus of elasticity of greater than 1,000,000 PSI(6892 MPa) by use of a plastic melt reinforced with loadings of about8-15% by volume of nanoparticles, with at least 70% of the nanoparticleshaving a thickness of 10 layers or less. As with the above describedembodiment, the plastic melt used has substantially the same materialcomposition as the article to be manufactured.

[0045] In this case of molding large parts with a modulus of elasticitygreater than 1,000,000 PSI (6892 MPa), it may be desirable to useengineering resins instead of polyolefins. Such engineering resins mayinclude polycarbonate (PC), acrylonitrile butadiene styrene (ABS), aPC/ABS blend, polyethylene terephthalates (PET), polybutyleneterephthalates (PBT), polyphenylene oxide (PPO), or the like. Generally,these materials in an unreinforced state have a modulus of elasticity ofabout 300,000 PSI -350,000 PSI (2068-2412 MPa). At these higher loadingsof nanoparticles (8-15% by volume), impact resistance will be decreased,but to a much lower extent than by the addition of the conventional25-60% by volume of glass fibers.

[0046] The invention may be used to reinforce any item ordinarilyproduced by thermoforming. For example, removable automobile hard topsdepicted in FIGS. 1 and 2, produced by plug assisted thermoforming maybe reinforced, using the inventive reinforcing particles. Suchthermoformed hard tops further comprising structural foams havingreinforcing nanoparticles exhibit better impact resistance,thermoinsulation and sound insulation than conventionally producedremovable automobile hard tops.

[0047] Reinforced child safety seats may also be manufactured accordingto the invention. Reinforcing nanoparticles of the invention canstrengthen the thermoformed shell of the seat as well as the foamcushioning within the seat. Child seats reinforced with nanoparticleshave better ductility for impact energy absorption than seats havingstandard reinforcing materials. The increased strength and impactresistance of such safety seats affords better protection for seatoccupants.

[0048] Reinforced articles having relatively thick walls may be producedaccording to the invention when the reinforced article comprises athermoformed skin blown from a reinforced polymer sheet under vacuumusing plug assistance. Larger, thicker, more complex articles may beformed according to the invention than is possible by blow moldingunreinforced polymers or polymers reinforced by, for example, glassfibers. This is because the reinforcing particles of the invention maybe evenly dispersed in molten resin, do not clump, and avoid generatingstress points likely to induce tears in the melted polymer during theblowing/forming step.

[0049] Although certain embodiments of the invention have been describedand illustrated herein, it will be readily apparent to those of ordinaryskill in the art that a number of modifications and substitutions can bemade to the blow molding system disclosed and described herein withoutdeparting from the true spirit and scope of the invention.

What is claimed is:
 1. A substantially hollow reinforced molded hard topfor an automobile, said hard top being produced by a method comprisingproviding a reinforced plastic sheet comprising at least onethermoplastic material and reinforcement nanoparticles dispersed withinthe at least one thermoplastic material, the reinforcement particlescomprising less than 15% of a total volume of the plastic sheet, atleast 50% of the reinforcement particles having a thickness of less thanabout 20 layers, and at least 99% of the reinforcement particles havinga thickness of less than about 30 layers; preheating said plastic sheet;communicating said preheated plastic sheet to a first mold assemblyhaving a first mold cavity defined by mold surfaces, the mold surfacescorresponding to a configuration of the article to be molded, an amountof the plastic sheet communicated to the first mold assembly beingsufficient to form a skin of the article; applying a vacuum to one sideof the first mold assembly while concurrently applying pressurized gasto an opposing side of the first mold assembly so as to force saidheated plastic sheet into conformity with the mold surfaces; cooling theconformed plastic sheet; transferring the conformed plastic sheet to asecond mold assembly; introducing to the conformed plastic sheet areinforced plastic melt made from material identical to or differentfrom that of the plastic sheet, said plastic melt having a blowing agentto achieve volume expansion and the production of a cellular reticulatestructure; cooling said plastic melt to form a solidified plastic memberadhered to said conformed plastic sheet, said conformed plastic sheetand said adhered solidified plastic member together comprising said hardtop; and removing said hard top from said second mold assembly, whereinthe hard top is filled with foamed insulating material formed from atleast one thermoplastic material and reinforcement nanoparticlesdispersed therein, and, wherein the at least one thermoplastic materialhaving reinforcement particles comprises about 2% to about 15% of atotal volume of the molded hard top, at least 50% of the reinforcementparticles having a thickness of less than about 20 layers, and at least99% of the reinforcement particles having a thickness of less than about30 layers.